1. Field of the Invention
The present invention relates generally to a drive source of a camera, and more particularly to a drive source of a two-step auto-focusing camera.
2. Description of Related Art
Conventionally, an auto-focus structure is used for controlling the telescopic movement of a lens of the camera. Auto-focus structures are used to automatically produce high-quality images with minimum user effort, and as such are important part of a modern camera.
The auto focus structure of the camera focuses on an object by analysis of the image on an image sensor. The image sensor is either a charge coupled device (CCD) sensor or a complementary metal oxide semiconductor (CMOS) sensor, and is controlled by a central processing unit (CPU) installed inside the digital camera. In the beginning, the lens moves back and forth around its original position (static position). As the lens moves, the CPU compares the image from the lens with the image in the image sensor. Finally, as the lens moves to the position where the image is the clearest the lens stops at that position. In this case, the lens has to be continuously driven back and forth by an annular cam. The annular cam is provided with a lens driving structure and driven to rotate by a stepping motor. The moment the CPU detects the clearest image as the lens moves back and forth, a stop signal is sent to the lens. Therefore, the lens stops in the best focal position (static position). When the lens is moved back to the original position, the annular cam starts to rotate again. Once the lens moves back to its original position, the photo interrupter installed in the lens cylinder uses the shutter at the end of the lens cylinder, to detect the original position of the lens. The shutter will interrupt the light of the penetration-type photo interrupter. The lens stops at the moment when the photo interrupter detects the original position.
Consequently, the lens in a digital camera auto focus system has to be driven continuously to obtain the clearest positions. However, narrow gaps usually exist between components of the camera when the components are assembled to form the camera, which, in most of cases, results in imprecise and unstable movement of the lens, which may finally result in losing the clearest position to obtain the best image.